Example: The PC's database could be downloaded to each of two Mobile Computers in the morning. Later in the day, one Mobile Computer is used to do an Add and the other Mobile Computer is used to do a Remove of that same inventory (or some portion of the quantity Added earlier.) The two Mobile Computers have no knowledge of each others' transactions, but when they've both been uploaded to the PC, the quantities will be correct once both sets of transactions have been accepted via Pending Uploads (a.k.a. the Upload from Mobile Devices function).
Tools>Options>Allow Negative Quanities on Mobile Devices makes it possible to Remove inventory using a Mobile Computer or PDT even though the PC's database showed a quantity of 0 at the time of the last Create Mobile Database